This invention relates generally to differentials and more particularly, to differentials capable of controlled power transmission to multiple output shafts.
The control of power output from a power source such as a drive shaft to more than one output (i.e., two wheels driven by respective output shafts) is a function accomplished in numerous ways in the prior art. A differential is typically used to transmit power to output shafts on opposing sides of the differential. For purposes of illustration only, the following discussion is with reference to a conventional differential having one input and two outputs in the form of co-axial output shafts driving respective wheels. However, it will be appreciated by one having ordinary skill in the art that the following discussion applies equally to other differential types having more outputs and having outputs which are different from shafts turning wheels.
In a conventional differential having two output shafts, rotational power is transmitted from a differential housing or ring gear to orbit one or more planet gears about a differential axis. The planet gears are engaged with side gears, each of which is connected to and turns an output shaft. When the planet gears orbit, they rotate the side gears to turn their respective output shafts. The planet gears normally do not rotate when the output shafts are equally loaded, and therefore the side gears rotate at the same speed. However, when the output shafts are loaded differently (e.g., when one wheel begins to spin faster than the other, when the traction of one wheel is significantly greater than the traction of the other, etc.), the planet gears rotate as they orbit, thereby turning the side gears at different speeds with respect to one another and driving the output shafts at different speeds. This speed differential can present problems in system performance, particularly when one output shaft spins with little or no load, thereby drawing power through the path of least resistance to that shaft.
Conventional differentials often employ limited slip devices to provide a minimum threshold torque resistance to each output shaft. When one wheel begins to slip and drain power supplied to the differential, a limited slip device on that shaft provides a minimum resistance. Power therefore continues to be transmitted to the opposite shaft rather than being drained to the shaft corresponding to the slipping wheel.
Most early differentials employ one or more elements which apply a constant force against the planet gears and/or the side gears to establish the above-mentioned minimum threshold torque resistance. These elements include pins which are threaded through the differential housing and which ride upon the rear surfaces of the gears, spring-loaded wedge blocks fitted between the planet gears and pressing with frictional engagement against the sides of the gears, and braking disks, disk packs, and/or Belleville springs pressed under spring force against one or more faces of the gears, etc. Although each of these elements function adequately to exert frictional force against the planet gears or side gears to inhibit spinning, they are either incapable of adjustment or must be adjusted manually after the equipment has been stopped. For example, the Belleville springs commonly used are often inaccessible without disassembling at least part of the differential, and have a spring force which is generally not adjustable. Also, the braking disks and disk packs are usually pressed by a conventional element (such as an adjustable threaded fastener or by an internal spring) which must be hand turned or adjusted as the disks or packs wear upon the gear surfaces. Such a differential design increases maintenance and operational costs of the differential, requires interruption of vehicle operation for adjustment, and results in a differential having braking effectiveness which varies as elements wear.
To address some of the drawbacks of older differentials, newer differential designs employ assemblies and devices for braking planet or side gears without manual adjustment and without the need to stop differential operation for adjustment. For example, U.S. Pat. No. 4,776,234 issued to Dennis W. Shea employs an energizing coil and magnet capable of adjusting pressure of a clutch pack against a side gear when the coil is energized. The electromagnetic control of this device enables a user to adjust the braking force of a desired gear through a range of frictional braking forces. As another example, U.S. Pat. No. 4,934,213 issued to Yoshikazu Niizawa uses oil pressure cylinders supplied by a controllable pressurized oil source to control pressure upon a frictional clutch having a number of frictional clutch plates between a differential housing and a side gear of the differential. By changing the oil pressure to the oil pressure cylinders, pressure upon the clutch plates can be changed to thereby change the frictional braking upon the side gear. Both the Shea and Niizawa devices represent improvements over the prior art in their ability to be adjusted without manually adjusting braking elements and without stopping the differential. But like other prior art devices capable of xe2x80x9con-the-flyxe2x80x9d gear braking adjustment, these devices are relatively complex, particularly in comparison to their earlier counterparts. Such devices are expensive to manufacture, assemble, service, and repair. These problems are due at least in part to the design necessary for the anticipated applications of the differentials. The differentials must be able to operate on equipment such as cars, trucks, and off-road vehicles, and must therefore operate under demanding conditions of stress, power, and speed. However, these differentials are not well suited to less demanding applications in which the vehicles are not fast moving and are not exposed to heavy load conditions.
In light of the problems and limitations of the prior art described above, a need exists for a differential which is simple, easy and inexpensive to manufacture, assemble, service, and repair, well-adapted to low speed and normal loading conditions, capable of traction control adjustment (i.e., braking of planetary and/or side gears) without stopping the differential, and which is preferably infinitely adjustable over a differential braking range. Each preferred embodiment of the present invention achieves one or more of these results.
In each of the preferred embodiments of the present invention, an actuation element is movable during differential operation either directly or indirectly by a user to exert a braking force upon a side gear or a planet gear of the differential. The actuation element is most often a element accessible by a user, but in some embodiments is the side or planet gear itself. The braking force upon the side gear or the planet gear can be exerted directly upon a gear surface by a braking element or surface or upon an element (such as a pivot or an axle) to which the side gear or the planet gear is mounted. In either case, the amount of braking force applied is preferably controllable by controlled actuation of the actuation element through a range of positions. Therefore, a user can control the amount of braking force applied as desired or in response to different vehicle operating conditions (e.g. running upon and along a slope, operating in slippery or muddy conditions, and the like).
In a number of preferred embodiments, the actuation element is a lever, thrust rod, magnetic coil, or similar element which is movable or energized either to drive a brake element in harder or lighter frictional engagement with a planet or side gear or to drive a planet or side gear in harder or lighter frictional engagement with a brake element. In the first case, the actuation element can be a lever of a band brake which is tightenable around plant gear pivots, brake blocks in wedging relationship between planet gears, cone clutch elements fitted to planet or side gears in a cone clutch arrangement, or can be a lever, thrust rod, or similar element movable to push, pull, or otherwise move an element into frictional engagement with a surface of a side gear, or can be a magnetic coil that can be energized to move a brake element into frictional braking engagement with a gear or pivot. In the second case, the actuation element can be a lever, thrust rod, or similar element connected to an axle of the differential and capable of axially moving the axle (and the side gear mounted thereon) toward and away from a brake element. In other embodiments, the actuation element can be cables or other linking members which can be tightened or loosened to either directly actuate a brake element (such as by being directly connected to a band brake about a vehicle axle) or indirectly actuate a brake element via movement of a lever, thrust rod, or similar element as mentioned above. In still other embodiments of the present invention, actuation of the brake elements is inherently generated by operation of the differential. Specifically, wheel slip or speed differentiation between the axles generates outward loading and movement upon the planet and side gears to engage brake elements beside the planet and side gears.
The brake element actuated by the actuator can also take a number of forms. For example, the brake element can be one or more brake pads which are located between the inside of a differential housing and the side or planet gears. When the side or planet gears are translated axially away from the center of the differential (such as by the outward loading and movement generated by wheel slip or axle speed differentiation described above, or by an actuation element forcing the side or planet gears toward the differential housing) the brake pads become sandwiched between a surface of the side or planet gears and the differential housing and become frictionally engaged to slow or even prevent gear rotation. In another embodiment of the present invention, the brake element can be a wedge-shaped brake block located between two planet gears. When an actuation element such as a band brake or a brake element located beside the orbit path of the brake block (and planet gears) is moved to press the brake block in an inward direction, the brake block wedges against the planet gears or their respective pivots to slow or stop their rotation. The brake elements can instead be cone clutch elements mating with the planet or side gears. Such brake elements can be forced through a range of contact with the planet gears in the same or similar way as the wedge-shaped brake blocks, or with the side gears by being cammed or threaded into engagement with the side gears. In still other preferred embodiments of the present invention, the brake element can be a band brake fitted around an axle extending from the differential, a surface or portion of an element thrust into abutting relationship with a face of the gear by the actuation element, or a friction element against which the gears are pressed when moved by the actuation element.
The embodiments of the present invention disclosed herein present viable solutions to traction control problems because they are directed to applications in inherently slow-moving vehicles. While these embodiments are generally not applicable to higher-speed applications (such as automotive or truck applications), their designs employ a minimal number of parts which are easily manufactured, assembled, and serviced and therefore create a significant advantage over much more complex and expensive prior art devices for low speed vehicles. As mentioned above, each of the brake and actuation element combinations preferably permit a range of braking forces to be exerted upon the planet or side gears through a range of actuation positions while the differential is operating. This adjustment flexibility not only provides a user with improved and xe2x80x9con-the-flyxe2x80x9d traction control, but also with much greater and more convenient control over the way in which the differential responds to slip in various operating conditions.
As discussed above, controlled traction differential devices exist for exerting braking force against one or more gears of a differential in a number of different ways. However, in addition to the advantages described above, each embodiment of the present invention offers a degree of traction control which is unavailable in much more complex and expensive prior art devices. In prior art differential traction control devices, not only is a user unable to quickly adjust gear braking according to a particular vehicle operation or environment encountered, but a user typically cannot make adjustments through a wide rangexe2x80x94even down to no gear braking and up to full gear lock. For example, in a riding lawn mower application where the riding lawn mower is driven relatively slowly and tightly in a circle (such as where a user is cutting grass around the base of a tree), the speed differentiation of the axles extending from the differential is high. However, it is not desirable in such a case to brake the differential gears. Conventional traction control differential devices are unable to distinguish between cases such as this where traction control is not desired and those vehicle operations or environments in which traction control and gear braking is desired. Therefore, gear braking either occurs too often (e.g., constant gear braking even when no speed differentiation exists between the axles) or not often enough, or is restricted to certain operational speeds or speed differences between axles. Such constraints define limitations of prior art traction control devices, and have invited partial solutions which invariably add significant complexity and cost to differentials.
In contrast, highly preferred embodiments of the present invention provide a user with a very large amount of control over differential traction control operationxe2x80x94both in the amount of gear braking exerted and in the vehicle operations and environments in which gear braking is performed. In contrast with operation of conventional traction control differentials, a user of the present invention can preferably select from an infinite range of gear braking forces, including no gear braking and braking causing full gear lock. Also, the user preferably has full control to vary the amount of braking forces exerted at any given time, in any given application or operating condition, and at any given differential operating speed or axle speed difference. Differential frictional losses are therefore low and system efficiency is relatively high. In addition (and in large part owing to the relatively slow differential applications to which the present invention is directed), the present invention offers the above-described control without employing complex or expensive assemblies or devices.
Further objects and advantages of the present invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.